Voltage regulator with a buffer + voltage divider

[rprodrigues]

Hi guys,


I do need your assistance again.

This time I plan to get 5v from 7v, and to do it I will use a simple voltage divider to get 5V from 7V and pass it trough a buffer (opamp), so I get it stable at the output side of the regulator (opamp).

Is this schema a power safe one or it would wast a lot of power?

How could I know that ?


Thank you

 

[Russ Hensel]

Sounds fine, but not sure I understand. A skematic would help.

 

[rprodrigues]

Sounds fine, but not sure I understand. A skematic would help.

Hi,

This is what i mean:

**broken link removed**


Do you know how can I discover what is the power efficiency of this circuit?

 

[Papabravo]

It is a really silly plan because the opamp is probably not designed to supply much more than 25 to 50 mA. If you want some idea of the wasted heat, take 5 from 12 and multiply by the load current. At 50 mA that would be about 350 mW of wasted heat. Depending on the package the opamp will sit there and cook. It might also go unstable and break into oscillation, or just sag down to some unpredictable level. You would be so much better off with an LDO regulator. Why in the name of ... are people so averse to using 3-terminal regulators? There just is no excuse or justification for all the nonsensical methods out there for regulating voltage without using a part designed for that purpose. Do we imagine that we are legends in our own minds and have just discovered the freaking holy grail?

 

[rprodrigues]

It is a really silly plan because the opamp is probably not designed to supply much more than 25 to 50 mA. If you want some idea of the wasted heat, take 5 from 12 and multiply by the load current. At 50 mA that would be about 350 mW of wasted heat. Depending on the package the opamp will sit there and cook. It might also go unstable and break into oscillation, or just sag down to some unpredictable level. You would be so much better off with an LDO regulator. Why in the name of ... are people so averse to using 3-terminal regulators? There just is no excuse or justification for all the nonsensical methods out there for regulating voltage without using a part designed for that purpose. Do we imagine that we are legends in our own minds and have just discovered the freaking holy grail?

Oh, men!

Your reply is great! Thank you!


Since my application is a battery supplied one, I guess that the problem about using a LDA regulator is its low power efficiency. Am I right?


Should I use a series diode to get about 6v and run it through a LDO do get a better efficiency, since the dropout would be 6V-5V instead of 7v-5v ?

 

[Papabravo]

Oh, men!

Your reply is great! Thank you!


Since my application is a battery supplied one, I guess that the problem about using a LDA regulator is its low power efficiency. Am I right?


Should I use a series diode to get about 6v and run it through a LDO do get a better efficiency, since the dropout would be 6V-5V instead of 7v-5v ?

No just use the LDO. Aim for a drop out voltage of 300 mV. If you put other components in the path the battery life will be reduced. What you want is to have the regulator provide the desired output for as long a time as possible before dropping out. In your example if you have a 5V LDO with 7V on the input, it will provide 5V until the input voltage drops to 5.3V.

If you stick a diode in the path then you add 0.7V to 0.3V and the output starts to drop when the input reaches 6V.

In terms of time it may not make much difference, since battery voltage remains relatively constant for 80% of its useful life and drops off like a brick in the rain at the end of its life. What you care about is not efficiency but battery life. Current drain is current drain; if you want to extend battery life you must reduce the current drain.

 

[AtomSoft]

Not my topic, but nice info!

 

[Russ Hensel]

Your circuit is an ok way to get from one reference voltage to a lower one. But not for a power supply. ( although I think I have seen a similar circuit for a low power - power supply splitter, that is a center ground from a normally single ended supply. )

 

[rprodrigues]

Guys,


I decided to go with an LM2675 from National!!!!

It is a switching regulator with an wonderful efficiency (about 86% ~ 90%).


Thank you very much!